Ubiquitin is a highly conserved 76 amino acid protein expressed in all eukaryotic cells. The levels of many intracellular proteins are regulated by a ubiquitin-mediated proteolytic process. This process involves the covalent ligation of ubiquitin to a target protein, resulting in a poly-ubiquitinated target protein which is rapidly detected and degraded by the 26S proteasome.
The ubiquitination of these target proteins is known to be mediated by the enzymatic activity of three ubiquitin agents. Ubiquitin is first activated in an ATP-dependent manner by a ubiquitin activating agent, for example, an E1. The C-terminus of a ubiquitin forms a high energy thiolester bond with the ubiquitin activating agent. The ubiquitin is then transferred to a ubiquitin conjugating agent, for example, an E2 (also called ubiquitin moiety carrier protein), also linked to this second ubiquitin agent via a thiolester bond. The ubiquitin is finally linked to its target protein to form a terminal isopeptide bond under the guidance of a ubiquitin ligating agent, for example, an E3. In this process, monomers or oligomers of ubiquitin are attached to the target protein. On the target protein, each ubiquitin is covalently ligated to the next ubiquitin through the activity of a ubiquitin ligating agent.
The enzymatic components of the ubiquitination pathway have received considerable attention (for a review, see Weissman, Nature Reviews 2:169-178 (2001)). The members of the E1 ubiquitin activating agents and E2 ubiquitin conjugating agents are structurally related and well characterized enzymes. There are numerous species of E2 ubiquitin conjugating agents, some of which act in preferred pairs with specific E3 ubiquitin ligating agents to confer specificity for different target proteins. While the nomenclature for the E2 ubiquitin conjugating agents is not standardized across species, investigators in the field have addressed this issue and the skilled artisan can readily identify various E2 ubiquitin conjugating agents, as well as species homologues (See Haas and Siepmann, FASEB J. 11:1257-1268 (1997)).
Generally, ubiquitin ligating agents contain two separate activities: a ubiquitin ligase activity to attach, via an isopeptide bond, monomers or oligomers of ubiquitin to a target protein, and a targeting activity to physically bring the ligase and substrate together. The substrate specificity of different ubiquitin ligating agents is a major determinant in the selectivity of the ubiquitin-mediated protein degradation process.
In eukaryotes, some ubiquitin ligating agents contain multiple subunits that form a complex called the SCF having ubiquitin ligating activity. SCFs play an important role in regulating G1 progression, and consists of at least three subunits, SKP1, Cullins (having at least seven family members) and an F-box protein (of which hundreds of species are known) which bind directly to and recruit the substrate to the complex. The combinatorial interactions between the SCF's and a recently discovered family of RING finger proteins, the ROC/APC11 proteins, have been shown to be the key elements conferring ligase activity to ubiquitin ligating agents. Particular ROC/Cullin combinations can regulate specific cellular pathways, as exemplified by the function of APC11-APC2, involved in the proteolytic control of sister chromatid separation and exit from telophase into G1 in mitosis (see King et al., supra; Koepp et al., Cell 97:431-34 (1999)), and ROC1-Cullin 1, involved in the proteolytic degradation of IκB″ in NF-κB/IκB mediated transcription regulation (Tan et al., Mol. Cell. 3(4):527-533 (1999); Laney et al, Cell 97:427-30 (1999)).
The best characterized ubiquitin ligating agent is the APC (anaphase promoting complex), which is multi-component complex that is required for both entry into anaphase as well as exit from mitosis (see King et al., Science 274:1652-59 (1996) for review). The APC plays a crucial role in regulating the passage of cells through anaphase by promoting ubiquitin-mediated proteolysis of many proteins. In addition to degrading the mitotic B-type cyclin for inactivation of CDC2 kinase activity, the APC is also required for degradation of other proteins for sister chromatid separation and spindle disassembly. Most proteins known to be degraded by the APC contain a conserved nine amino acid motif known as the “destruction box” that targets them for ubiquitin ubiquitination and subsequent degradation. However, proteins that are degraded during G1, including G1 cyclins, CDK inhibitors, transcription factors and signaling intermediates, do not contain this conserved amino acid motif. Instead, substrate phosphorylation appears to play an important role in targeting their interaction with a ubiquitin ligating agent for ubiquitin ubiquitination (see Hershko et al., Ann. Rev. Biochem. 67:429-75 (1998)).
Two major classes of E3 ubiquitin ligating agents are known: the HECT (homologous to E6-AP carboxy terminus) domain E3 ligating agents; and the RING finger domain E3 ligating agents. E6AP is the prototype for the HECT domain subclass of E3 ligating agents and is a multi-subunit complex that functions as a ubiquitin ligating agent for the tumor suppressor p53 which is activated by papillomavirus in cervical cancer (Huang et al. (1999) Science 286:1321-1326). Members of this class are homologous to the carboxyl terminus of E6AP and utilize a Cys active site to form a thiolester bond with ubiquitin, analogous to the E1 activating agents and E2 conjugating agents. However, in contrast, the members of the RING finger domain class of E3 ligating agents are thought to interact with an ubiquitin-conjugated-E2 intermediate to activate the complex for the transfer of ubiquitin to an acceptor. Examples of the RING domain class of E3 ligating agents are TRAF6, involved in IKK activation; Cb1, which targets insulin and EGF; Sina/Siah, which targets DCC; Itchy, which is involved in haematopoesis (B, T and mast cells); IAP, involved with inhibitors of apoptosis; and Mdm2 which is involved in the regulation of p53.
The RING finger domain subclass of E3 ligating agents can be further grouped into two subclasses. In one subclass, the RING finger domain and the substrate recognition domain are contained on different subunits of a complex forming the ubiquitin ligating agent (e.g., the RBx1 and the F-box subunit of the SCF complex). In the second subclass of ubiquitin ligating agents, the ligating agents have the RING finger domain and substrate recognition domain on a single subunit. (e.g., Mdm2 and cb1) (Tyers et al. (1999) Science 284:601, 603-604; Joazeiro et al. (2000) 102:549-552). A further class of ligating agents are those having a “PHD” domain and are homologs of the RING finger domain ligating agents (Coscoy et al. (2001) J. Cell Biol. 155(7):1265-1273), e.g., MEKK1. The PHD domain ligating agents are a novel class of membrane-bound E3 ligating agents.
Mdm2 belongs to the second subclass of single subunit E3 ligating agents and is involved in regulating the function and stability of p53, an important tumor suppressor. In cells, p53 functions as a DNA-binding transcription factor which induces the expression of genes involved in DNA repair, apoptosis, and the arrest of cell growth. In approximately 50% of all human cancer p53 is inactivate by deletion or mutation. The level of p53 in the cell is maintained at low steady-state levels, and is induced and activated post-translationally by various signal pathways responsive to cellular stress (Lakin et al. (1999) Oncogene 18:7644-7655; Oren, M. (1999) J. Biol. Chem 274:36031-36,034). Stimuli that trigger the stress response and activate p53 include oxygen stress, inappropriate activation of oncogenes and agents that cause damage to DNA (e.g., ionizing radiation, chemicals, and ultra violet light).
The carboxyl terminus of Mdm2 contains a variant of the RING finger domain (Saurin et al. (1996) Trends Biochem. Sci. 21:208-214) that is critical for the activity of this E3 ligating agent. Recent studies have shown that Mdm2 mediates the ubiquitination of itself resulting in the formation of poly-ubiquitin chains on the protein (Zhihong et al (2001) J.B.C. 276:31,357-31,367; Honda et al. (2000) Oncogene 19:1473-1476; Shengyun et al. (2000) 275:8945-8951). Further, the ubiquitin ligating activity of Mdm2 is dependent on its RING finger domain.
Typically, the ubiquitination of target proteins by E3 in cells results in the formation of poly-ubiquitin chains. An isopeptide bond is formed between the carboxyl terminus of the ubiquitin and the ε-amino group of Lys in the target protein. The extension or formation of ubiquitin chains results from the formation of additional isopeptide bonds with the Lys48 (and sometimes Lys63) of a previously conjugated ubiquitin and the carboxyl-terminal Gly of an additional ubiquitin. The efficient recognition of a ubiquitinated target protein by a proteosome requires at least four ubiquitins linked in this configuration. However, in the case of Mdm2-mediated ubiquitination of p53, neither Lys48 or Lys63 is involved in the formation of poly-ubiquitin chains. Recent studies show that human Mdm2 mediates multiple mono-ubiquitination of p53 by a mechanism requiring enzyme isomerization (Zhihong et al. (2001) J. Biol. Chem. 276:31,357-31,367). Further, in vitro, the transfer of ubiquitin to p53 can occur independent of E1 when using an E2 pre-conjugated with ubiquitin. These results suggest that the pre-conjugated E2 can bind to Mdm2 and thereafter transfer the ubiquitin to the Mdm2 in the absence of an E1.
Thus, ubiquitin agents, such as the ubiquitin activating agents, ubiquitin conjugating agents, and ubiquitin ligating agents, are key determinants of the ubiquitin-mediated proteolytic pathway that results in the degradation of targeted proteins and regulation of cellular processes. Consequently, agents that modulate the activity of such ubiquitin agents may be used to upregulate or downregulate specific molecules involved in cellular signal transduction. Disease processes can be treated by such up- or down regulation of signal transducers to enhance or dampen specific cellular responses. This principle has been used in the design of a number of therapeutics, including phosphodiesterase inhibitors for airway disease and vascular insufficiency, kinase inhibitors for malignant transformation and Proteasome inhibitors for inflammatory conditions such as arthritis.
Due to the importance of ubiquitin-mediated proteolysis in cellular process, for example cell cycle regulation, there is a need for a fast and simple means for identifying ubiquitin agents that are catalytic components of this enzymatic pathway, and for identifying agents that modulate the activity of these catalytic components. Thus, an object of the present invention is to provide methods of assaying for ubiquitin agents that are catalytic components of ubiquitin-mediated proteolysis and, more particularly, methods of assaying for agents that modulate the activity of the ubiquitin agents.